This invention relates to a process for manufacturing carbon fiber pellets, the resulting pellets and the use of the pellets in the production of reinforced thermoplastic resins.
Carbon fiber reinforced thermoplastic resins are conventionally manufactured by batch blending carbon fiber with thermoplastic resin and feeding the blend to an extruder/compounder. The resulting extrudate is formed into molded articles employing conventional injection molding or extrusion molding techniques. Developments in the compounding industry have recently favored feeding the carbon fiber separately from the resin pellets/powder. The carbon fiber is typically introduced into the polymer melt in an effort to minimize the mechanical degradation of the fiber length. Separate feeding of the carbon fiber requires precise metering of the carbon fiber addition. Economic concerns for high volume production require a free-flowing carbon fiber product form. Conventional carbon fiber forms are manufactured by cutting a carbon fiber strand made up of carbon filaments bundled together with a sizing agent into short lengths, e.g., 3-10 mm or so. The cutting operation necessarily results in blunt-ended forms, e.g., as shown in U.S. Pat. No. 4,818,615. Similar forms are described in U.S. Pat. No. 5,227,238 and EPA 338,919.
The flow characteristics of the various carbon fiber product forms have a significant impact on the feeding and metering behavior of the fiber. Two of the principal factors influencing the flow characteristics of a given carbon fiber product form (other than its dimensions) are its density and its shape. For optimum flow characteristics, a carbon fiber product form should have a fairly high bulk density and relatively low flow-resistance, i.e., a streamlined shape. Product forms produced by a strand-cutting technique such as described in aforementioned U.S. Pat. Nos. 4,818,615 and 5,227,238 and EPA 338,919 might possess a suitably high level of bulk density but their blunt-ended configurations are not conducive to low flow-resistance. Although it is known from Japanese Patent No. 4,300,353 that a spherical carbon fiber pellet can be obtained by mixing carbon fibers with a solution of polymer as a sizing agent in an apparatus having a tumbling and flowing action, any advantage in flow properties owing to the streamlined shape of the pellet is more than offset by its low density which is on the order of 0.05-0.15 g/cc (50-150 g/l). Thus, the spherical pellets described in Japanese Patent No. 4,300,353, indicated to be useful therein as insulating material, reinforcement for carbon material, filters and adsorption equipment, would not be suitable for use in conventional reinforced thermoplastic resin manufacturing operations.
Heretofore, no carbon fiber pellet manufacturing process has been able to provide a product which at the same time possesses high density and a streamlined morphology, characteristics which combine to yield low resistance to flow. A process capable of producing such a product and, of course, the product itself, would represent a significant advance in the technology of carbon fiber reinforcements for thermoplastic resins.